Fluorescent dyes have many uses and are known to be particularly suitable for biological applications in which the high detectability of fluorescence is required. Fluorescent dyes are used to impart both visible color and fluorescence to other materials. Many applications utilize chemically reactive fluorescent dyes by chemically attaching the dye to reactive sites on a wide variety of materials such as cells, tissues, proteins, antibodies, enzymes, drugs, hormones, lipids, nucleotides, nucleic acids, or natural or synthetic polymers to make fluorescent conjugates. With these synthetic probes, ligands are frequently used to confer a specificity for a biochemical reaction that is to be observed and the fluorescent dye provides the means of detection or quantitation of the interaction.
Fluorescence useful for such applications is generally initiated by absorption of light from an external, relatively concentrated light source. The sensitivity of these applications is improved by having dyes that have high absorbance of the exciting light and high fluorescence quantum yield. The applications are furthermore improved by having dyes that resist photobleaching by the exciting light and that have spectral wavelengths in a range that avoids the background from contaminants that may be present in the samples. For many biological applications it is useful to have dyes whose fluorescence is not quenched by water, since most biological measurements are made in aqueous solution.
Certain lasers are particularly useful as a concentrated light source for the excitation of fluorescence. The argon laser has been the most common light source for excitation of fluorescence, with principal output at 488 nm and 514 nm. Now other lasers are increasingly used, such as helium-neon lasers that can be selected to have maximum output at either 543 nm, 594 nm, or 633 nm; the krypton laser which has significant output at 568 nm and 647 nm; and light emitting diodes which are available at this time with output commonly above 660 nm; resulting in increased demand for longer wavelength fluorescent dyes.
A number of dyes that have previously been found to be fluorescent do not have significant absorbance at desired longer excitation wavelengths. Many also have other characteristics which interfere with or limit their usefulness. For example, many known fluorescent dyes are significantly quenched in aqueous solution or are unstable during the illumination.
Dyes derived from dipyrrometheneboron difluoride have many desirable characteristics. Simple alkyl derivatives of the fluorophore 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene have been described by Treibs & Kreuzer, Difluorboryl-komplexe von di- und tripyrrylmethenen, LIEBIGS ANNALEN CHEM. 718,208 (1968) and by Worries, Kopek, Lodder, & Lugtenburg, A novel water-Soluble fluorescent probe: Synthesis, luminescence and biological properties of the sodium salt of the 4-sulfonato-3,3',5,5'-tetramethyl-2,2'-pyrromethen-1,1'-BF.sub.2 complex, RECL. TRAV. CHIM. PAYS-BAS 104, 288 (1985) as being highly fluorescent with spectral properties that are similar to fluorescein, with maximum absorbance at about 490 to 510 nm and maximum emission at about 500 to 530 nm. U.S. Pat. No. 4,774,339 to Haugland et al. (1988) ('339 patent) describes 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (dipyrrometheneboron difluoride) dyes including hydrogen, halogen, alkyl, cycloalkyl, aryl, arylalkyl, acyl, and sulfo-substituted derivatives that contain reactive groups suitable for conjugation to biomolecules, that have good photostability, and which have fluorescein-like spectra. As described in the '339 patent, and by Pavlopoulos, et al., Laser action from a tetramethylpyrromethene-BF.sub.2 complex, APP. OPTICS 27, 4998 (1988), the emission of the alkyl derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorescent dyes clearly overlaps that of fluorescein. The overlap allows the alkyl derivatives of dipyrrometheneboron difluoride to be used with the same optical equipment as used with fluorescein-based dyes without modification of the excitation sources or optical filters. As a result of having the same spectral characteristics, however, the fluorescence of the known class of alkyl-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes is not readily suitable for detection in combination with fluorescein or for use in applications where excitation by longer wavelength sources such as the helium-neon or krypton lasers or light emitting diodes is required.
Although the '339 patent discloses some dyes with an absorption maximum of greater than 525 nm (Table 5), the '339 patent is neither enabling nor prior art for the invention of the subject long wavelength reactive derivatives of dipyrrometheneboron difluoride dyes. Of the longer wavelength dyes listed, all were measured in chloroform which slightly enhances the spectral absorption and shifts the emission maxima to longer wavelength by about 10 nm (compared to methanol). The '339 patent also discloses a non-reactive dye having an absorption maximum greater than 600 nm (cmpd. 27) which has been found to have an undesirably low quantum yield and very broad absorption and emission spectral band widths.
U.S. Pat. No. 4,916,711 to Boyer, et al. (1990) ('711 patent) discloses a method of using derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dyes, in particular symmetrical alkyl and sulfonated alkyl derivatives, as laser dyes. The '711 patent also discloses a multitude of possible alternatives for substituents of the basic tricyclic structure which can be used for the patented method. The '711 patent, however, is neither enabling nor prior art for the invention of long wavelength reactive derivatives of dipyrrometheneboron difluoride dyes. The dyes described in the '711 patent are not reactive dyes. In addition, the '711 patent neither recognizes nor recites the effect or advantage of substituents that enhance the long wavelength fluorescence properties of such dyes.
Inventors Kang and Haugland have filed two co-pending applications on long wavelength dyes that are non-reactive: HETEROARYL-SUBSTITUTED DIPYRROMETHENEBORON DIFLUORIDE DYES AND THEIR SYNTHESIS (Ser. No. 07/629,596, filed Dec. 18, 1990 now U.S. Pat. No. 5,248,782) and ETHENYL-SUBSTITUTED DIPYRROMETHENEBORON DIFLUORIDE DYES AND THEIR SYNTHESIS (Ser. No. 07/704,287, filed May. 22, 1991 now U.S. Pat. No. 5,187,288). In addition, other materials incorporating long wavelength dipyrrometheneboron difluoride dyes have been disclosed in co-pending applications: FLUORESCENT FATTY ACIDS DERIVED FROM DIPYRROMETHENEBORON DIFLUORIDE DYES (Ser. No. 07/654,881, filed Feb. 13, 1991 by Haugland & Kang now U.S. Pat. No. 5,338,854) and DIPYRROMETHENEBORON DIFLUORIDE LABELED FLUORESCENT MICROPARTICLES (Ser. No. 07/629,466, filed Dec. 18, 1990 by Brinkley, Haugland & Kang, pending). None of the long wavelength dipyrrometheneboron difluoride dyes described in the references are the chemically reactive dyes of this invention.
The novel dyes described in this invention contain one or more substituent groups coupled to the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene ("BDI") fluorophore resulting in significant absorbance and fluorescence at desired longer wavelengths. The dyes of this invention also have the chemical reactivity necessary for conjugation to the reactive sites commonly found in biomolecules, drugs, and natural and synthetic polymers. The reactive dyes of this invention are combined with a variety of materials to form novel dye conjugates. The novel dye conjugates are desirable for use in combination with conjugates formed from other fluorescent dyes such as fluorescein or alkyl-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dyes in that the fluorophore of the novel conjugates can be both selectively excited and detected because of their spectral shift to longer wavelengths, particularly an absorption maximum at greater than 525 nm and an emission maximum at greater than 550 nm. The new reactive dyes have the advantage over other dyes that absorb at these wavelengths, of being electrically neutral, photostable and being, in most cases, highly fluorescent in both organic and aqueous solution with relatively narrow absorption and emission spectra.